This invention relates to a method of operating a two-stage hydraulic machine, and more particularly, a method of operating a two-stage hydraulic pump-turbine operated at a constant speed under a steady operating condition in which movable guide vanes are disposed at the respective pressure stages which are connected in series with each other by a return passage.
Generally, in a single-stage pump-turbine, a plurality of movable guide vanes are annularly disposed between a runner chamber and a vortex chamber of a spiral case, and a valve is located at an inlet portion of the vortex chamber on the upstream side thereof. The amount of water flowing through the runner can be controlled by adjusting the degree of opening of guide vanes after the inlet valve has been completely opened. This controlling method is very suitable for and easily applicable to the operation of a single-stage hydraulic pump-turbine to increase hydraulic efficiency and operational stability of the pump-turbine.
However, in a multistage hydraulic pump-turbine in which respective stages are connected in series by a return passage, it is considerably difficult to arrange guide vanes and a mechanism for operating them on each stage because of constructional and operational problems. Therefore, actually, in a known multistage hydraulic pump-turbine, the flow amount is controlled by adjusting the opening of the inlet valve located upstream of the spiral case.
Recently, requirements for a multistage hydraulic pump-turbine have been increased because of the requirements for installing a pumped storage power plant at a position having a high head. Particularly, it is required to develop a method of operating a pump-turbine of a small number of stages (i.e., two-stage) in which the movable guide vanes and an operating mechanism thereof are relatively easily arranged on the outer periphery of the runner of each stage.
FIG. 1 shows a typical two-stage hydraulic pump-turbine operated at a constant speed, in which a runner 2 of a high pressure stage and a runner 3 of a low pressure stage are axially located with space on a common shaft of the pump-turbine. The upper runner 2 is covered with upper and lower covers 4 and 5 to define a runner chamber 8 of the high pressure stage, and the lower runner 3 is covered with upper and lower covers 6 and 7 to define a runner chamber 9 of the low pressure stage. Both runner chambers 8 and 9 are connected with each other by a return passage 10. On the outside of the runner chamber 8 is located a spiral case 11 provided with a vortex or scroll chamber 12 communicated with the runner chamber 8. An inlet portion of the vortex chamber 12 is connected to a penstock 14 through an inlet valve 13. A plurality of movable guide vanes 15 and 16, the degrees of openings of which are constructed to be adjustable, are annularly disposed at the outer peripheral portions of the runners 2 and 3 of the high and low pressure stages, respectively. The guide vanes of each stage are operatively connected to an operating mechanism, not shown, through a control device to operate the pump-turbine under the normal condition.
The two-stage hydraulic pump-turbine constructed as described above is operated as a turbine in the following manner.
When fed from the penstock 14 to the centrifugal casing 11 through the inlet valve 13 passes successively through the movable guide vanes 15, the runner 2, the return passage 10, the movable guide vanes 16, and the runner 3. The water is then discharged into a tailrace, not shown, through a draft tube 17. On the other hand, when the hydraulic pump-turbine is operated as a pump at the same speed as that at a time when it operates as a turbine, the water pumped up by the runner 3 flows from the draft tube 17 to the penstock 14 through a course reverse to that described above.
Total hydraulic characterisics of the two-stage hydraulic pump-turbine may be given by combining hydraulic characteristics of the respective pressure stages. Therefore, in order to grasp problems of a two-stage hydraulic pump-turbine, it may be better to discuss in advance about problems regarding hydraulic characteristics of a single-stage hydraulic pump-turbine.
With the single-stage hydraulic pump-turbine, FIG. 2 shows curves representing the relationships between efficiencies on each opening degree of the guide vanes and unit rotating speeds of N/.sqroot.H.sub.t and N/.sqroot.H.sub.p when the pump-turbine is operated as a turbine and a pump, respectively, wherein N designates revolution per minute (rpm) of the pump-turbine, H.sub.t (m): the turbine net head and 72 t: the turbine efficiency when operated as a turbine, and wherein the H.sub.p (m) designates effective pumping head and .eta.p: the pump efficiency when the pump-turbine is operated as a pump.
As can be understood from FIG. 2, the rotating speed N/.sqroot.H.sub.to at the most effective point as a turbine does not accord with the rotating speed N/.sqroot.H.sub.po at the most effective point as a pump, and N/.sqroot.H.sub.po is always larger than N/.sqroot.H.sub.to, which fact is an inevitable problem on hydraulic characteristics of a single speed reversible type hydraulic pump-turbine.
In an actual operation of a hydraulic pump-turbine, between these unit rotating speeds there is the following relation. ##EQU1## This relation shows the fact on the hydraulic characteristics that the rotating speed N/.sqroot.H.sub.po at the most effective point as a pump is considerably different from the rotating speed N/.sqroot.H.sub.to at the most effective point as a turbine. Namely, in a pumped storage power plant operated at a predetermined normal water level, when it is determined that the hydraulic pump-turbine is operated as a pump at a rotating speed under the most or substantially the most effective condition, it must be operated as a turbine at the same rotating speed under a condition having a lower hydraulic operating efficiency.
In the followings, let us describe problems on the hydraulic characteristics under the normal operating condition of a two-stage hydraulic pump-turbine provided with runners having equal outer diameters (i.e., D.sub.1 =D.sub.2 in FIG. 1) at the respective stages by taking into consideration the discussion about the single-stage hydraulic pump-turbine as described hereinbefore and with reference to FIG. 3.
In FIG. 3, H.sub.1 designates the turbine net head in a high pressure stage, H.sub.2 : the turbine net head in a low pressure stage, Q: the water flow amount, Q.sub.0 : the water flow amount in a case when operated at the normal water level (normal condition O), a.sub.0 : the degree of opening of the guide vanes at each stage under the condition O, a.sub.n (N=1, 2, 3, . . . ): the degree of opening larger than a.sub.0 under another condition, a-n (n=1, 2, 3, . . . ): degree of opening smaller than a.sub.0 under another condition, and .DELTA..eta.: the relative efficiency difference with respect to the highest turbine efficiency. In FIG. 3 an abscissa represents a flow amount ratio Q/Q.sub.0 and an ordinate represents effective head ratios H.sub.1 /H.sub.10 and H.sub.2 /H.sub.20 in the high and low pressure stages, respectively. Thus, FIG. 3 shows the relation between hydraulic characteristics of the effective head and flow amount. Thus, the total turbine net head of a two-stage hydraulic pump-turbine can be obtained by adding the turbine net heads of the respective stages.
Under the normal condition O (in FIG. 3) as a turbine in which the respective stages are operated under hydraulically equivalent conditions by fully opening the inlet valve 13, the following relations hold with reference to the turbine net heads of the respective stages. ##EQU2## where H.sub.10 and H.sub.20 are turbine net heads of the high and low pressure stages and H.sub.0 is the total turbine net head.
However, it should be noted that the normal condition O of each pressure stage is within an operating area considerably apart from the most effective condition (i.e., .DELTA..eta.=0) when the pump-turbine is operating as a turbine on the low head side. In this area, the pump-turbine is operated as a turbine at an increased unit rotating speed and with a low hydraulic efficiency and a water separation phenomenon is caused, thereby inducing secondary local flow resulting in the cavitation, vibration and noise. Moreover, the water separation phenomenon and the secondary local flow may often generate at a time when the pressure on the outlet side of the runner is lower than that on the inlet side thereof, and with a two-stage hydraulic pump-turbine, a pressure on the outlet side of the low pressure stage is considerably lower than that on the outlet side of the high pressure stage (corresponding to the inlet side of the low pressure stage). Therefore, it is important to determine how to stably control the operation of the low pressure stage of a two-stage hydraulic pump-turbine operating under the steady condition as a turbine.